High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations are generally prepared by conventional melt polymerizations. Solid state polymerization is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers during the polymerization phase is eliminated. Thermal degradation is also essentially avoided during the solid state portion of the polymerization.
The low molecular weight polyester prepolymers utilized in solid state polymerizations are generally in the form of pellets or chips. Such pellets can vary greatly in size; however, as a general rule, the smaller the size of the pellets of polyester prepolymer the faster the solid state polymerization will proceed. Very fast rates of solid state polymerization can be attained by utilizing polyester prepolymers which are in the form of porous pills as described in U.S. Pat. No. 4,755,587 to Rinehart.
Most thermoplastic polyesters, including polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), produced by melt-phase polymerization are almost completely amorphous in nature. Such amorphous polyester prepolymers which are prepared by melt polymerization are normally converted from the amorphous state to the crystalline state prior to solid state polymerization to raise their sticking temperature. This is done to keep pellets or chips of the polyester prepolymer being solid state polymerized from sticking together as a solid mass.
When an amorphous polyester is heated from ambient temperature to above its glass transition temperature (Tg), it will become sticky before it starts to crystallize. The sticking temperature of an amorphous polyester is usually about 20.degree. C. above its Tg. The crystallization rate of the polyester will not be fast enough to be practical until its temperature is further raised to about 30.degree. C. above its sticking temperature. To achieve the maximum crystallization rate, the temperature of the polyester must be raised even higher. For example, PET has a Tg of 74.degree. C. and a sticking temperature of about 95.degree. C. The crystallization rate of PET is rather low until the temperature is raised to above 125.degree. C. and in practice, PET is usually crystallized at temperatures between 150.degree. C. and 190.degree. C.
PEN is a relatively new polyester with promising properties for fiber and packaging applications. PEN has a Tg of about 118.degree. C. and a crystalline melting point (Tm) of 268.degree. C. It exhibits a crystallization peak between 180.degree. C. and 220.degree. C. Its sticking temperature is about 140.degree. C. when in the amorphous state. According to conventional wisdom, the best crystallization temperature range for PEN would be between 180.degree. C. and 220.degree. C.
In the crystallization process, the polyester must undergo a sticky stage. This takes place in the period between the time the polyester temperature exceeds the sticking temperature and the time the polyester becomes well crystallized. Therefore, commercial-scale crystallizers for continuous crystallization of polyesters must provide vigorous agitation to prevent agglomeration or lumping of the polyester pellets. Two types of continuous crystallizers have been widely used, namely, agitated vessels and fluidized beds.
Heretofore, in the continuous crystallization process of particulate polyesters, PET in particular, the polyester pellets at ambient temperature without any pretreatment are directly charged into the crystallizer in which the heat transfer medium (e.g., hot air, hot nitrogen, or hot oil) maintains a suitable crystallization temperature. Under appropriate operating conditions, the polyester pellets can be crystallized without lumping or agglomeration.
However, when PEN pellets are exposed to the crystallization conditions as determined by conventional wisdom, the pellets undergo a sudden and rapid expansion as they are heated to near the crystallization temperature. The puffed up skins of the pellets are very sticky and, within seconds, the pellets agglomerated tightly into big lumps, vigorous agitation notwithstanding. This indicates that the conventional crystallization process is not suitable for commercial crystallization of PEN.